Current MOS transistors, both planar and three-dimensional (such as FinFETs) use high-κ dielectrics as the gate oxide layer together with a very thin interlayer (IL), typically SiO2. These high-κ dielectrics are used in conjunction with layers of a conductor that are part of the overall gate structure. The work functions of the conductor layers are chosen to provide the right threshold voltages for both n-channels and p-channels. As device size continues to shrink, the reliability of transistors under normal operation has become increasingly problematic. Issues include bias temperature instability (BTI), wherein the transistor characteristics change as a result of voltages applied to the metal gate, and hot electron instability (hot carrier injection or HCI) wherein the transistor characteristics change as a result of injection of energetic carriers from the channel. Many of these reliability issues are connected with interface states at the boundary between the interface layer and the underlying semiconductor substrate (e.g., silicon, germanium, or silicon-germanium). The problems arise as a result of dangling Si and/or Ge atoms at the interface. Typically, after cleaning, the surface atoms are passivated by weakly bound H atoms which are easily displaced or removed, resulting in re-activation of the interface.
One approach to stabilizing the dangling Si or Ge atoms is to attach F atoms. Fluorine binds strongly to both silicon and germanium, and once bonded, the interface becomes stable. For example, Jain et al. demonstrated improved hot carrier lifetime for CVD deposited WF6 and SiH4 compared with sputter deposited WSix (Jain, V., et al. 1991 VLSI Technology, Digest of Technical Papers, 1991 Symposium, 28-30 May 1991 VLSI Technology Inc., CA, 91-92). The estimated lifetime for the fluorine-doped WSix was nearly double the lifetime of the sputter deposited WSix without fluorine. The improved hot carrier lifetime was attributed to the stronger Si—F bond compared to the Si—H bond.
However, introducing fluorine to the interface is challenging. Typically, exposure to fluorine-containing gases provides a means of introducing F atoms to the interface, but the control over the amount of delivered fluorine is imprecise. Excess fluorination of the interface has the effect of creating additional dielectric layers that increase the effective dielectric thickness. It is also possible to use ion implantation methods, but these methods tend to damage other materials in the transistor such as the gate dielectric, and they are not suitable for use with three-dimensional structures, because they are highly directional and cannot provide uniform implantation of F atoms.